1. Field of the Invention
The present invention relates to an optical recording and reproducing apparatus for optically recording signals on a recording medium and reproducing signals carried on a recording medium, in which a light source such as a laser is used. More specifically, the present invention relates to a tracking control system for controlling a light beam to accurately scan on the tracks of a recording medium.
2. Description of the Related Art
In recent years, as one way to realize a thin optical recording and reproducing apparatus or to facilitate rapid scanning by the apparatus, it has become known to conduct tracking by moving a movable portion (i.e. optical head) composed of optical elements that have been separated from the optical system of the apparatus instead of moving the whole optical system. Furthermore, there has been devised a construction in which a galvano-mirror is used as the tracking actuator disposed in a stationary portion so as to reduce the weight of the movable portion.
Hereinafter, a conventional tracking control system will be described with reference to FIG. 21, which is a block diagram showing a construction for a conventional tracking control system.
As is shown in FIG. 21, a light beam 108 emitted from a light source 101 such as a laser diode is collimated by a collimating lens 102, and thereafter goes through a beam splitter 103 so as to be reflected by a galvano-mirror 130 serving as a fine tracking actuator. The light beam reflected by the galvano-mirror 130 is further reflected by a mirror 104 disposed in a movable portion, and is thereafter converged on a rotating disk 107 by means of an objective lens 105. The disk 107 is connected to a spindle motor 106, which rotates the disk 107.
The light beam is reflected by the disk 107, led through the objective lens 105, and thereafter is reflected by the mirror 104 and the galvano-mirror 130 in the respective order so as to be incident on the beam splitter 103. The light beam incident on the beam splitter 103 is reflected toward and goes through a convex lens 109, and is thereafter split into light beams 111 and 115 by means of a cylindrical polarized beam splitter 110 (hereinafter referred to as a "cylindrical PBS").
The split light beam 111 is converged on a photodetector 112 consisting of several light-sensitive portions, one of which is further divided into two portions. The split light beam 111 is, more specifically, converged on the two divided portions of the photodetector 112. Then, two outputs A and B from the two divided portions are input to corresponding terminals of a differential amplifier 114. Each of the outputs A and B corresponds to the amount of light converged on each of the two divided portions. The differential amplifier 114 conducts an operation for obtaining the difference between the outputs A and B so as to produce a tracking error signal (hereinafter referred to as a "TES").
Such a method for obtaining a TES, in which the TES is defined as a difference between outputs of the differential amplifier 114, is known as a push-pull method, as is disclosed in Japanese Laid-Open Patent Publication No. 49-60702, for example.
The outputs A and B from the two divided portions are also input to a summing amplifier 116. The summing amplifier 116 conducts an operation for obtaining a sum total of the outputs A and B so as to produce a total light amount signal. As for the TES output from the differential amplifier 114, it is input to a variable amplifier 117. The gain of the variable amplifier 117 is so adjusted that the amplitude of the TES is roughly constant at point a. An output from the variable amplifier 117 is input to a divider 118, to which the total light amount signal from the summing amplifier 116 is also input. Accordingly, the divider 118 divides the output from the variable amplifier 117 by the output from the summing amplifier 116, so that the amplitude of the TES is roughly constant against changes in the amount of light during a recording or erasing operation and changes in the reflectance of the disk 107.
The other split light beam 115 from the cylindrical PBS 110 is also converged on the photodetector 112, but it is converged on another light-sensitive portion which is further divided into four portions. A focus error signal (hereinafter referred to as a "FES"), which indicates misconvergence of the light beam incident on the disk 107, is obtained based on outputs from the four divided portions of the photodetector 112. Detection of the FES is conducted by a differential amplifier 113 through an astigmatic method, which is a known technique. Based on the FES, a known focus servo control is conducted by driving a focusing actuator (not shown) so that the light beam incident on the disk 107 is so converged as is prescribed. Since the present invention does not directly relate to focus servo systems, descriptions therefor are omitted.
Next, operation of an optical system as a whole, in cases where tracking servo control is conducted so that the light beam converged by the objective lens 105 is radiated accurately on a given track, will be briefly described with reference to FIG. 21. In the tracking control system shown in FIG. 21, the tracking servo control is conducted by driving the galvano-mirror 130 serving as a fine tracking actuator and/or a linear motor 131 serving as a coarse tracking actuator. The galvano-mirror 130 is driven mainly in response to a TES having a high frequency, while the linear motor 131 is driven mainly in response to a TES having a low frequency. The linear motor 131 is also driven when conducting a search over the whole area of the disk 107, that is, when the light spot travels over the entire disk area.
A TES whose amplitude has been made roughly constant against changes in the light amount of the light beam and changes in the reflectance of the disk 107 by means of the divider 118, as was described earlier, is then input to a phase compensation circuit 121. An output from the phase compensation circuit 121 is input to a driving circuit 128, which drives the galvano-mirror 130. The galvano-mirror 130 is driven so as to rotate by an output from the driving circuit 128, which output corresponds to the TES. As the galvano-mirror 130 rotates, the direction in which the light beam is directed is altered, the light spot traveling across the tracks on the disk 107 so as to be located upon the sought track. (Hereinafter, the direction across the tracks on an optical disk is referred to as the "tracking direction" thereof.) Thus, the light spot is controlled so as to be always located right on the sought track.
The linear motor 131 is capable of moving from the inner periphery to the outer periphery of the disk 107 in the tracking direction thereof. The mirror 104 and the objective lens 105 are mounted on the linear motor 131 so as to form an optical head as a whole. The light spot can travel from the inner periphery to the outer periphery of the disk 107 in the tracking direction in accordance with the movement of the linear motor 131. When tracking servo control is conducted, the output of the phase compensation circuit 121 is input to another phase compensation circuit 123 by way of an equivalent filter circuit 122. The phase compensation circuit 123 is used for controlling the linear motor 131. The equivalent filter circuit 122 has characteristics similar to the input-output characteristics, i.e. input-rotation characteristics, of the galvano-mirror 130 serving as the fine tracking actuator. An output from the phase compensation circuit 123 is input to a driving circuit 129, which drives the linear motor 131. Thus, the linear motor 131 is controlled in such a way that the galvano-mirror 130 rotates from a natural state as a reference posture, the natural state meaning a state where the center axis of the light beam incident on the objective lens 105 coincides with the optical axis of the optical system.
However, in such a construction as that shown in FIG. 21 where the galvano-mirror 130 is used as the fine tracking actuator and is provided in a stationary portion, and the optical head is composed of the linear motor 131 movable in the tracking direction, the mirror 104, and the objective lens 105, the optical path defined as the distance from the galvano-mirror 130 by way of the mirror 104 to the objective lens 105 is long. If the posture of the galvano-mirror 130 in an uncontrolled state (i.e. natural state) changes from an initial state in the direction in which the galvano-mirror 130 gravitates or in the direction in which the galvano-mirror 130 rotates, a deviation of the center axis of the light beam from the optical axis of the optical system (hereinafter, this deviation is referred to as "disagreement of optical axes) occurs. A longer optical path tends to increase the scale of the problem. In other words, a longer path increases an offset that a TES is made to have due to such disagreement of optical axes.
Disagreement of optical axes due to change in posture of the galvano-mirror 130 is better illustrated in FIG. 22. As is seen from FIG. 22, when the galvano-mirror 130 rotates counterclockwise, as would be necessitated when tracking servo control is conducted, the disagreement of optical axes occurs, causing the TES to have an offset. Disagreement of optical axes occurs whenever the galvano-mirror 130 is shifted from the initial state shown in FIG. 22. In other words, the disagreement of optical axes occurs even when no servo control is conducted, if the posture of the galvano-mirror 130 is altered because of gravity, etc., as is mentioned above. Such disagreement of optical axes, whether occurring when tracking servo control is conducted or not, causes the TES to have a large offset due to such factors as the spherical aberration of the objective lens 105, the coma of the light beam, and/or an eclipse due to the frame of the lens. If tracking servo control by rotating the galvano-mirror 130 is conducted in such a case, the TES is made to have an added offset.
If the disagreement of optical axes when no tracking servo control is conducted is large, the TES is bound to have a large offset. It may be impossible to remove such an offset in a circuit disposed after the TES is detected, if the TES is so large that the circuit is saturated. Moreover, the amplitude of the TES may become small, or the TES may not appear at all because of an eclipse due to the frame of the objective lens, etc. In cases where the TES does not appear, it is impossible to make it appear even by removing the offset thereof. Furthermore, if the TES has an offset when tracking servo control is conducted, the light spot on the disk 107 is prevented from being located right on the sought track. That is, the light spot on the disk 107 is in an off-track state.
Such an off-track state due to rotation of the galvano-mirror 130 during tracking servo control invites the problem of deterioration of the recording characteristics when information is recorded on the disk 107 and/or deterioration of the reproduction characteristics when information carried on the disk 107 is reproduced, since the light spot is likely to get off the track, that is, the tracking accuracy is deteriorated. Moreover, if deterioration with age and/or change in thermal circumstances should change the posture of the galvano-mirror 130 when no tracking servo control is conducted, thereby causing disagreement of optical axes, the TES may have a large offset, or the TES may not appear at all in extreme cases. In such cases, it becomes impossible to lead the tracking servo control system into a stable operation when initiating tracking servo control. That is, the whole tracking servo control system cannot be started, greatly undermining the reliability of the system.